1. Field of the Invention
The present invention relates to a method for extending the gas lifetime of excimer lasers.
2. Description of the Background
Excimer lasers represent an extension of laser technology into the ultraviolet portion of the spectrum. Excimer lasers offer the capability for pulsed short ultraviolet wavelength systems with very high peak power. An excimer is a compound that has no stable ground state and exists as a bound molecule only in electronically excited states. Many excimer lasers utilize the noble gases, which generally do not form stable chemical compounds. For example, the krypton fluoride laser is a prime example of an excimer laser. In such a laser, a gas mixture containing krypton and fluorine is irradiated with high energy electrons to produce the metastable excited state of KrF* excimer which is temporarily bound. The molecule dissociates according to the reaction: EQU KrF.fwdarw.Kr+F+h.upsilon.
As there is no stable ground state, a population inversion is readily produced. Due to the nature of the reaction, excimer lasers are generally pulsed devices, with pulse durations on the order of nanoseconds.
Excimer lasers are now available commercially. Commercial excimer lasers require gas mixtures consisting of rare gases, such as He, Ne, Ar, Kr or Xe, and halogen donors, such as F.sub.2, NF.sub.3 or HCl. The particular components of the gas mixture used depend upon the particular lasing transition of interest. XeCl, KrF, ArF and XeF are examples of lasing transitions used today. However, XeCl, KrF and ArF, which operate at 308, 248 and 193 nm wavelengths, respectively, are the most widely used.
For KrF operation, the rare gases used in the laser chamber are Kr diluted in He or Ne along with a halogen donor which can be either F.sub.2 or NF.sub.3. NF.sub.3 is a better energy acceptor, but F.sub.2 is used in all commercial excimer lasers today because post-discharge recombination kinetics are more favorable for F.sub.2 in terms of minimizing gas degradation. The same applies to ArF with the exception that Ar replaces Kr, and ArF operation is more susceptible to gas degradation.
Excimer lasers are unlike any other gas laser as they generally operate with a fixed volume of gas which needs replacing often enough to make it mandatory for the user to either refill the laser chamber or purify and replenish the halogen donor. The need to replenish gas mixtures for excimer lasers is a result of undesirable chemical reactions occurring inside the laser chamber. As a consequence of these reactions, the gas mixture changes during operation of the laser and the laser output decreases. The characteristic feature of such gas degradation is loss of halogen donor and formation of gaseous impurities. Despite improvements in laser design to minimize gas degradation, there continues to be strong interest in extending the gas lifetime of excimer lasers. There are two important reasons for this. First, and most importantly, there is a need to minimize downtime of laser operation. Second, there is a need to reduce gas consumption of the expensive rare gases such as Ne, Kr and Xe.
Over the years, there have been a number of attempts to extend the continuous operational period of excimer lasers. In one method, partial gas replacements are performed during laser operation. In this case, the gas mixture in the laser is simply replaced slowly, in discrete but small steps, while maintaining operation. This method does eliminate laser downtime, however, it does not reduce gas consumption, and it adds to the gas handling hazards as cylinders of toxic halogen donor gas must remain open.
A second method involves adding small amounts of halogen donor gas during laser operation. While this approach can effectively replace halogen donor, it does not remove impurities in the gas which limit the useful gas lifetime.
Several methods are known for removing impurities in excimer lasers. Many of these methods utilize metal getter systems and molecular sieves which work on removing all gaseous species in the mixture except rare gas. In order to take advantage of the purified gas using such a method, the halogen donor is simply replaced prior to flowing this gas back into the laser chamber so that gas consumption is limited to halogen donor only. Devices utilizing this principle are commercially available, however, they are extremely expensive and require complex gas handling. A more economical approach, and thus most common in the field, entails the use cryogenic purification of the laser gas. This methodology takes advantage of the fact that many of the impurities resulting from gas degradation can be removed in a low temperature trap. To be sure, gas lifetimes of excimer lasers have been extended considerably using this technique on-line during KrF and ArF operation.
Unfortunately, the use of cryogenic purification on KrF operation, for example, is limited in that the lowest temperature allowable for on-line use is about -180.degree. C. below that at which one begins to reduce sufficient Kr in the gas mixture, which decreases laser output. This results in an inability to remove an important impurity, CF.sub.4, from the laser chamber, which limits the gas lifetime of KrF operation when cryogenic purification is used. For ArF, a cryogenic trap can be used at lower temperatures of around about -196.degree. C., which is sufficient to condense out more CF.sub.4. However, at such lower temperatures, a higher cooling capacity is required from the cryogenic trap.
Thus, a need continues to exist for a method of extending the gas lifetime of excimer lasers in an effective, but economic manner.